Many technologies require the removal of a solute from a liquid or solid medium for the purposes of purifying the carrier medium or concentrating the solute for recovery or analysis. Concentration of the solute is carried out by intimately exposing the carrier medium to a solvent, which may be either liquid or gas but which is most frequently liquid. When the carrier is a solid and the solvent is a liquid, the process is referred to as solid-liquid extraction. Likewise, when the carrier medium is a liquid, the process is referred to liquid--liquid extraction.
Conventional methods of liquid--liquid extraction involve mixing a first liquid phase, consisting of a feed liquid contaminated with a solute, with a second liquid phase, consisting of a solvent. The first and second liquid phases are immiscible and selected to have a density and polarity difference to enable separation of the two liquid phases after contact. The process yields an "extract", i.e., the solvent and extracted solute, and a "raffinate", i.e., the treated liquid feed having a reduced concentration of solute.
Conventional methods of liquid--liquid extraction can be carried out on a continuous or batchwise basis. One established technique for contacting the two liquid phases involves counter current flow of the second liquid phase through the first liquid phase, or vice versa. The solvent stream passes through the feed stream in a single pass, but may thereafter be recycled for additional passes. Contact between the feed liquid and the solvent during each pass in such a "linear" operation is relatively low. Alternately, the two liquid phases can be more intimately contacted by mechanically agitating a vessel containing the two liquid phases. While such a method offers improved contact, it is limited in applicability due to the requirement for mechanical agitation, and can only be carried out on a batchwise basis.